The invention relates to a propulsion control system for motor vehicles to the type shown in U.S. Pat. No. 4,484,280.
It concerns a wheel slip control (ASR) for the driven wheels of a vehicle having a driven axle, a slip comparator being assigned to each of these wheels. If one driven wheel exceeds a given slip value, this wheel is braked. If both driven wheels exceed this slip value, output torque of the vehicle engine is reduced. In one variation, the torque is reduced if only one drive wheel is spinning.
Because of the fixed slip threshold value, a wheel slip control occurs earlier than required. As a result of the differing rotational speeds of the vehicle wheels during turning, these slip values are reached much earlier than in the case of straight-ahead driving. Also, the road condition and the relationship between the road and the vehicle are not taken into account.
It is therefore the objective of the invention to improve the known system to the extent that the slip threshold values are adapted better to the momentary conditions, the control is refined and as a result the safe driving range is expanded.
According to the invention, this objective is achieved by providing several slip threshold comparators having different threshold values and selecting one of the slip threshold comparators by a logic circuit as a function of lateral acceleration and control signals of the propulsion system.
While the fixing of the slip threshold value in the case of the known system represents a compromise between an optimal propulsion force and the best cornering force or lateral control force, the slip threshold values in the case of the system according to the invention, during straight-ahead driving (low lateral acceleration), are selected for an optimum propulsion force, and during cornering (moderately high to high lateral acceleration while taking the driving speed into account), said slip threshold values are selected for a good cornering force or lateral control force.
The lateral acceleration signal is used as the control signal for the fixing of the thresholds and results in an effect on the logical processes which will then change the control means for affecting the braking and engine torque. In comparison to a fixed slip threshold, this represents a significant improvement of the system.
The signals obtained in the logic circuit may, in a simple way, be used for switching an additional all-wheel drive (driving of the wheels of an additional axle) and will thus further increase safety. Advantageously, these signals may also be used for varying the speed of the rising of the brake pressure for the wheel brakes and the adjusting speed of the power control element so that the intervention carried out by the system with respect to the brakes and the drive becomes less noticeable.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.